Handsets which are standard equipment in telephone and other telecommunication stations such as cellular terminals consist basically of a receiver and a transmitter microphone connected by a handgrip. The utility of the conventional handset in a noisy environment such as a construction site, a machine shop, an airport, a bus terminal, or a car telephone station is diminished due to their passing much of the interfering ambient noise to the user's ear. Accordingly, expedients such as a volume control sometimes are used in handsets of the prior art to improve incoming speech intelligibility by increasing the incoming sound signal relative to the noise signal level. The same unwanted noise still passes to the user, however, and the resulting signal-to-noise ratio still substantially interferes with the speech intelligibility.
Although handsets of the prior art typically have not employed noise-cancelling circuits, such circuits may be found, for example, in prior art headsets for aircraft use. Active noise-cancelling circuits typically pick up the ambient noise signal with a noise-cancelling microphone and create an inverted version of the unwanted signal that is applied to the receive channel where it subtractively interferes with the noise signal. The difficulty, however, in designing a handset which incorporates electronic cancellation techniques, has to do in part with the acoustic properties of a handset and specifically with matching of the phase and amplitude of the noise-cancelling signal to the noise itself. The handset must be designed to have an amplitude and phase response that is compatible with a practical control circuitry for the noise-cancelling operation.